log(N) = 0.093 + 3.389 log(CR) (1) (r2=0.764, t34=10.35, p<0.001), or 147.8 friends

What’s that wierd formula in the title? It’s the best-fit reduced major axis regression equation between neocortex ratio and mean group size for the sample of 36 primate genera taken from Robin Dunbar’s 1992 paper, and which is normally interpreted as being a natural limit on human group size of 150 (or 147.8 to be exact). I learned about Dunbar’s number, as it’s called, during a course I did with Dave Snowden, who also blogged about it here. Both Dunbar’s paper, and Snowden’s blog, make for fascinating reading, especially for the people I know who question what optimum team size is, and what the biggest possible project is that you can coordinate successfully. But why am I blogging about it here? That answer’s quite simple.

I had kept myself happily free of all the Web 2.0 pseudo-social networking sites up until now, but chose to sign up for Facebook to get in contact with some old friends I couldn’t reach otherwise. Even so, I didn’t want to spend more time and energy there than was absolutely necessary to maintain these important social contacts. I had heard various horror stories about Facebook. My father told me, “what monster have I created? It just keeps sucking me in deeper and deeper.” My dear friend Evelyn told me that she got quite a lot of friendship invitations from unknown men simply because, as some of them wrote: “your profile picture’s so hot”. (n.b. she’s since changed it to one of her dog).

Every time you go to your home page in Facebook, it tells you how many “friends” you have. Are these really friends? Maybe some of them are, but many (if not most) are people who’ve become connected to you simply because of the social network construction mechanism available within Facebook makes it so easy, and because of the competitive aspect: “I’ve got more friends than you”. (Thought: could a lack of virtual friends lead to a virtual depression?)

Having all these contacts as friends, though, is an expensive proposition. The cost of maintaining social ties can be considerable. In one of his blog posts, Christopher Allen (quoting Dunbar) points out the consequences that a social network of 150 would have:

The group size predicted for modern humans by equation (1) would require as much as 42% of the total time budget to be devoted to social grooming.

42% is a lot of time! No wonder many companies block people’s access to such sites at work. In an interesting corollary, Dunbar wrote the following:

My suggestion, then, is that language evolved as a “cheap” form of social grooming, so enabling the ancestral humans to maintain the cohesion of the unusually large groups demanded by the particular conditions they faced at the time.

Having a large network gives rise to a problem know as the Diplomat’s Dilemma: how do I maximize the usefulness of my network whilst minimizing the time I spend grooming it? I don’t know how others do it, but if I had to spend 42% of my time taking care of my social network, I’d never get any work done! In addition, not only the time involved, but also the type of effort involved, may vary greatly within one and the same network.

Snowden also points this out in his blog:

Now the assumption in Dunbar’s working and subsequent writing is that this level of knowledge requires physical proximity. However we now live in virtual as well as physical worlds so the nature of interactions change. The natural limit is probably in place, but its form, and the nature of its creation will have new variants for a new environment.

The cost of maintaining social ties is also analysed in a recent paper by Petter Holme and Gourab Ghoshal entitled “The Diplomat’s Dilemma: Maximizing Power for Minimal Effort in Social Networks”.

So, how do I solve the Diplomat’s Dilemma? Well, I set up a Facebook rule for myself. If I don’t know you well enough to invite you to my house, possibly contributing funding to help cover your travel costs, and am not comfortable with leaving you alone in my house, I won’t link to you on Facebook. I may exchange emails with you, or chat with you on some IM service, or may link to you on one of the professional social networking sites I’m on, but I’m not going to use Facebook for the 147.8 “friends” or acquaintances I have, but rather for the 15 “deep trust” friends (see Snowden’s blog for an explanation of that number).

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